Catalytic nanoporous membranes

ABSTRACT

A nanoporous catalytic membrane which displays several unique features including pores which can go through the entire thickness of the membrane. The membrane has a higher catalytic and product selectivity than conventional catalysts. Anodic aluminum oxide (AAO) membranes serve as the catalyst substrate. This substrate is then subjected to Atomic Layer Deposition (ALD), which allows the controlled narrowing of the pores from 40 nm to 10 nm in the substrate by deposition of a preparatory material. Subsequent deposition of a catalytic layer on the inner surfaces of the pores reduces pore sizes to less than 10 nm and allows for a higher degree of reaction selectivity. The small pore sizes allow control over which molecules enter the pores, and the flow-through feature can allow for partial oxidation of reactant species as opposed to complete oxidation. A nanoporous separation membrane, produced by ALD is also provided for use in gaseous and liquid separations. The membrane has a high flow rate of material with 100% selectivity.

This patent application claims the benefits of U.S. Provisional PatentApplication No. 60/503,668, filed on Sep. 17, 2003.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant tocontract number W-31-109-ENG-38 between the U.S. Department of Energyand the University of Chicago representing Argonne National Laboratory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a novel nanoporous membrane catalyst and aprocess of production of the nanoporous membrane catalyst, and moreparticularly, this invention relates to a nanoporous catalyst of highreactant conversion rates and a process of production of the catalystusing atomic layer deposition (ALD).

2. Background of the Invention

Selective catalytic oxidation (SCO) remains one of the most desirableand elusive technologies for chemical and fuels processing andenvironmental protection. Industrial catalysts are generally highsurface area substrates onto which an active dimensions of 1 nanometer(nm) to 20 nm and are often referred to as nanoparticles.

Nanoporous catalytic materials, predominantly in the form of zeolites,have gained wide acceptance as industrial catalysts for oil refining,petrochemistry, and organic synthesis, particularly for molecules withkinetic diameters below 1 nm.

There has been little prior work in which nanoporous alumina has beenused as a catalyst or as a support for catalytic nanoclusters. Rather,the anodic alumina, as a film or thin shell on an aluminum base, wasused directly as a catalyst. subsequently, oxide catalysts weresupported on these anodic alumina films, but these works did not employmembrane catalysts.

The use of anodic alumina membranes for deposition of metalnanoparticles into the pores from colloidal solution has been shown in:T. Hanaoka, H. P. Kormann, M. Kroell, T. Sawitowski, G. Schmid,Three-Dimensional Assemblies Of Gold Colloids In Nanoporous AluminaMembranes, Eur. J. Inorg. Chem., 807-812 (1998). Similarly, the use ofAAO materials for anchoring metal complexes on the pore walls was shownin: P. Braunstein, H. P. Kormann, W. Meyer-Zaika, R. Pugin, and G.Schmid, Strategies For The Anchoring Of Metal Complexes, Clusters, AndColloids Inside Nanoporous Alumina Membranes, Chem. Eur. J. 6, 4637-4646(2000). However, the membranes employed (250 nm pore diameter) were wellbeyond the nanoscale target dimensions necessary to effect reactionswith 1 nm to 10 nm size molecules.

U.S. Pat. No. 6,740,143 awarded to Corbin, et al. on May 25, 2004discloses a method for the synthesis of nanoporous carbon membranes. Themethod entails pyrolysis of selected polymers on porous substrates toproduce thin mixed matrix carbon film with pores. The carbon filmfacilitates the separation of small molecules from a reaction liquor.

U.S. Pat. No. 6,471,745 awarded to Foley, et al. on Oct. 29, 2002discloses catalytic membranes comprising highly-dispersed,catalytically-active metals in nanoporous carbon membranes and asingle-phase process to produce the membranes.

U.S. Pat. No. 4,921,823 awarded to Furneaux, et al. on May 1, discloseda method for the synthesis of nanoporous carbon membranes. The methodentails the discloses a porous anodic aluminum oxide membrane catalystsupport with pore sizes of 80 nm.

None of the aforementioned patents discloses a nanoporous membrane withpore sizes at least as small as 10 nm and a process for making suchnanoporous membranes. In addition, none of these patents discloses amethod for creating catalysts of vanadia with nanopores.

A need exists in the art for nanoporous membranes with pore sizes atleast as small as 10 nm and a process to make them. Such materials canprovide a higher degree of reaction selectivity and a greater complexityand range of catalysts including nanoporous vanadia catalysts.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel nanoporousmembrane catalyst and a process of production of the nanoporous membranethat overcomes many of the disadvantages of the prior art.

Another object of the present invention is to provide a process forformation of a nanoporous membrane on a substrate using ALD. A featureof the invention is that pores of anodized aluminum are reduced to sizesless than or equal to 10 nm by the deposition of metal oxide films withALD. An advantage of the invention is that electronic structure ofmaterials changes in the 1 nm to 10 nm range.

Still another object of the present invention is to provide a processthat uses a combination of anodic aluminum oxidation and ALD. A featureof the invention is that porous catalysts can be fabricated which allowfor atomic level control of both the pore wall diameters and pore wallcomposition. This feature allows for the uniform coating of the entirelength of the pore's inner surface. An additional feature of theinvention is the ability to add layers of a variety of materials,allowing a large number of different types of catalysts to be generated.An advantage of the invention is that the method can provide uniquecatalyst environments.

Yet another object of the present invention is to provide a processwhich can produce or form catalytic layers in which the pore diameterreaches sub-nanometer dimensions. A feature of this invention is thatmolecular-size selectivity can be an important factor in chemicalreactions and separations. An advantage of this feature is thatselective activation of bonds, e.g., terminal C—H bonds in alkanes, viamolecular size selection can be achieved.

Still another object of the present invention is to provide a processwhich can produce a nanoporous separation membrane. A feature of thisinvention is that AAO pore sizes can be reduced by the addition ofmultiple layers using ALD until all the pores are brought to the sameconsistent and uniform diameter. An advantage of this feature is thatthe membrane can be made selective for a particular molecular size bycontrolling the size of reactants (ALD precursors) which coat the insideof pores. An additional advantage of this feature is that it aids in theachievement of uniform catalytic results throughout the membrane.

Yet another object of the present invention is to provide a processwhich can give shorter contact times between a catalyst and reactant(s).A feature of this invention is that AAO pores can go all the way throughthe entire thickness of the membrane to create a flow-through catalyticmembrane. An advantage of this feature is that the reactant(s) have verybrief contact with catalytic material and thus heightens productselectivity, e.g., partial oxidation as opposed to total oxidation.

Briefly, the invention provides a process for making a membrane catalysthaving pores at no greater than 10 nm diameters, the process comprisingdepositing alternating monolayers of different precursor moieties upon asubstrate; allowing the monolayers to react with each other to form afirst film upon the substrate; depositing a second group of alternatingmonolayers of the different precursor moieties upon the first film; andallowing the monolayers from the second group to react and form a secondfilm upon the first film.

The invention also provides a nanoporous support material for acatalyst, the nanoporous support material comprising a film depositedupon anodic aluminum oxide (MO) via atomic layer deposition (ALD).

In addition, the invention provides a nanoporous catalyst, thenanoporous catalyst comprising a layer deposited upon a nanoporoussupport material via atomic layer deposition (ALD).

Further, the invention provides a nanoporous separation membranecomprising multiple layers of film deposited upon anodized aluminumoxide (AAO) via atomic layer deposition (ALD).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention together with the above and other objects and advantageswill be best understood from the following detailed description of thepreferred embodiment of the invention shown in the accompanying drawing,wherein:

FIG. 1 is a schematic diagram of a binary reaction sequence, inaccordance with features of the present invention;

FIG. 2A is a schematic diagram of a SEM image of anodized aluminum oxide(AAO) material before being coated; and

FIG. 2B is a schematic diagram of a SEM image of the same anodizedaluminum oxide (AAO) material after being coated with 15 nm of alumina(Al₂O₃).

DETAILED DESCRIPTION OF THE INVENTION

The inventors have produced catalytic membranes that display severalunique features, including higher specificities than conventionalcatalysts. The invention uses anodic aluminum oxide (AAO) membranes asthe catalyst substrate. Other suitable substrates include Vanadiacatalysts. Supported vanadia catalyst systems are excellent systems forinvestigating the effect of nanostructure on the activity andselectivity of oxidation catalysts, and offer the prospect of developinga new generation of optimized selective oxidation catalysts.

Where AAO substrates are used, the substrate is subjected to AtomicLayer Deposition (ALD), which allows the controlled narrowing of poresin the substrate by deposition of whatever material is required forcatalysis in atomic layers on the inner surface of the pores. Inaddition to the anodic alumina membranes, tubular ceramic membranes suchas the asymmetric gamma-alumina tubular membranes which are commerciallyavailable and have a high conductance also may be utilized in this ALDdeposition scheme.

The inventors have found that the pores in the substrate can be narrowedfrom 40 nm to less than or equal to 10 nm using this method. This poresize (referred herein as “nanoporous”) is of great interest in catalysisas changes in the electronic structure of materials occur in the 1 nm to10 nm scale. If desired, the pores can traverse completely through theAAO membrane to the other side to provide a flow-through catalyst.

Initially, the pores can have a nonconstant or varying diameter alongtheir lengths. However, the pore diameters can be made constant, andconsistently the same, by the deposition of multilayers via ALD.

Anodic aluminum oxide (AAO)-materials are produced electrochemically atArgonne National Laboratory and are also available commercially. AAOmaterials have organized arrays of channels with pore diameters as smallas 20 nm, pore densities exceeding 1×10¹¹ cm⁻² and lengths as long as 70microns. Selective regions of the channels, or the entire channel, canbe repeatedly coated with nearly any pure or mixed-metal oxide(including alumina), carbide, nitride or metallic film. This coatingprocess is performed using ALD techniques to assure uniform pores ofarbitrary diameter and composition. AAO pore diameters can be narrowedfrom their starting diameter of 20 nm to 200 nm to dimensions as smallas ˜1 nm. This size range encompasses much of the most interestingcatalytic dimensionality. For instance, the inventors surmise that thepore wall electronic structure change significantly as a function ofpore diameter in the 1 nm to 10 nm range.

By manipulating processing conditions during the manufacture of AAOmembranes, the diameter of the cylindrical MO pores can be chosen to beanywhere between 30±1 nm and 200±3 nm. ALD can be used to produce amembrane structure with ultra-uniform pore sizes with an extremelynarrow pore size distribution, within 0.1 nm. The ALD process iscontinued until the smallest reactants (ALD precursor moieties) can nolonger pass through the shrinking pores of the AAO membrane, therebysetting a lower limit on pore size. However, the ALD process continuesin the larger-diameter pores until all the pores constrict to the samediameter, resulting in an extremely narrow pore size distribution.Furthermore, the ultimate size of the pores can be controlled bychoosing the size of the precursor moieties which coat the inside of thepores. Thus, one can fine-tune the pore size of a particular AAOmembrane to be the size of a specific molecule to be separated byselecting ALD precursors of suitable size.

The inventors have found that using a combination of anodic aluminumoxidation (AAO) and atomic layer deposition (ALD) affords a facile,flexible route to the synthesis of ultra-uniform heterogeneous catalyticmembranes. The inventors further found that the ALD process coats theentire length of the pore inner surface uniformly to the same coatingthickness, not only on an individual pore's inner surfaces, but in allpores and nonporous surfaces of the support surface. This capabilityallows atomic level control of both the pore wall diameters and porewall composition.

An additional feature of the invention is the ability to add layers of avariety of materials, allowing a large number of different types ofcatalysts to be generated. Thus, the invention is versatile in terms ofthe catalysts that the invention can produce.

These materials are membranes which offer unique catalyst environmentswhich: 1) provide larger pores than conventional mesoporous materials(for containing large clusters or arrays of catalyst sites, forefficient in-diffusion of large/elaborate molecular precursors orfeedstock molecules, and for out-diffusion of large/elaborate productmolecules); 2) permit tailoring of channel size and wall composition byALD (including channel surfaces with hydrophobic, or hydrophilicnature); 3) constrain catalyst mobility, thus hindering agglomeration;and 4) control flow of reagents in and out of the catalyst.

The inventors have found that nanoporous materials can be made by usinga reactor which provides a continuous and viscous flow of inert carriergas to transport precursor reactant moieties to the sample substrates,and to purge or sweep the unused precursor reactant moieties out of thereaction zone.

Film deposition on substrates is also feasible using short-durationpulses of pure precursor moiety gases without any inert carrier gas.

The amount of precursor moiety gas required is empirically determined tosaturate the active sites on the substrate surface. The saturation aidsto insure dense, smooth, pinhole-free films, which are defect-free andcontinuous. The deposition is self-limiting. Once a monolayer of onemoiety is formed, additional exposure to that same moiety results in nosubstantial additional deposition. Only exposure to a second moiety,which is reactive towards the first moiety deposited on the substratesurface, results in the allowance and creation of any additional surfaceactivity. This process is repeated and multiple layers are formed uponthe substrate.

Synthesis of the nanoporous catalytic membranes begins with theproduction of Anodic Aluminum Oxide (AAO) membranes producedelectrochemically. The membranes have highly aligned, uniform-diameterpores from 20 to 400 nm in diameter and membrane thicknesses of 0.5microns to 70 microns.

To deposit a preparatory layer upon the AAO substrate, two differentprecursor moieties are needed. One of the two different precursormoieties is selected from the group consisting of titanium tetrachloride(TiCl₄), titanium (IV) isopropoxide (Ti(i-C₃H₇)₄), tin (IV) chloride(SnCl₄), tetraethyl tin (Sn(C₂H₅)₄), trimethyl aluminum ((CH₃)₃Al), andsilicon tetrachloride (SiCl₄). The other of the two different precursormoieties is selected from the group consisting of water, oxygen (O₂),ozone (O₃), hydrogen peroxide (H₂O₂), isopropanol (CH₃CHOHCH₃), and air.Multiple layers of these two precursor moieties are laid down upon thesubstrate and each other in the ABAB fashion described supra.

The preparatory film that results from these two different precursormoieties is selected from the group consisting of aluminum oxide(alumina/Al₂O₃), titanium (IV) oxide (titania/TiO₂), tin (IV) oxide(stannia/SnO₂) and silicon (IV) oxide (silica/SiO₂).

For deposition of a catalyst layer within the pores, one of the twodifferent precursors needed is selected from the group consisting ofvanadium (V) isopropoxide oxide (V(i-C₃H₇)₃O), chromium (III) acetylacetonate (Cr(C₅H₇O₂)₃), nickel (II) acetyl acetonate (Ni(C₂H7O₂)₂),vanadyl acetyl acetonate (VO(C₂H₇O₂)₂), bis(cyclopentadienyl) ruthenium(Ru(C₅H₅)₂), and methylcyclopentadienyltrimethyl platinum((C₆H₇)Pt(CH₃)₃).

The other of the two different precursor moieties is selected from thegroup consisting of water, oxygen (O2), ozone (O₃), hydrogen peroxide(H₂O₂), isopropanol (CH₃CHOHCH₃), and air.

In the aforementioned manner, multiple layers of these two differentprecursor moieties are laid down in a successive ABAB . . . fashion inwhich the A and B moieties alternate. Each pair of A and B layers reactto form multiple layers of the same film material. The nanoporouscatalytic layer .that results from the reaction of these differentprecursor moieties is selected from the group consisting of vanadium (V)oxide (vanadia/V₂O₅), chromium oxide (CrO_(x)), nickel (Ni), platinum(Pt), and ruthenium (Ru).

A salient feature of the instant invention is that the catalytic layeris deposited uniformly on the inner surfaces of the pores of thenanoporous support material and allows for greater catalyticselectivity.

Another salient feature is uniform and consistent pore size which givesconsistent performance through all dimensions of each catalyst producedby the instant invention.

An additional salient feature of the invention is the production of ananoporous separation membrane comprising multiple layers of filmdeposited upon anodized aluminum oxide (AAO) via atomic layer deposition(ALD). The AAO membrane initial pore sizes can be reduced to the size ofa particular molecule to be separated. The resulting membrane can beused in gaseous and liquid separation.

Deposition Protocol

Atomic layer deposition (ALD) is a thin film growth technique that usesa binary reaction sequence of alternating, saturating reactions betweengaseous precursor molecules and a substrate to deposit films in alayer-by-layer fashion. This is described in: S. M. George, A. W. Ott,J. W. Klaus, Surface Chemistry for Atomic Layer Growth, Journal ofPhysical Chemistry 100, 13121-13131 (1996).

ALD relies on the gaseous diffusion of precursor moiety molecules toreach all regions of the substrate. This quality, combined with theself-limiting surface chemistry that terminates after the completion ofthe deposition of each monolayer, allows substrates with extremely highaspect ratios, such as cylindrical objects, to be coated thoroughly anduniformly. This feature has been put to great advantage, e. g.,production of conformal coatings on AAO membranes as is shown in: J. W.Elam, D. Routkevich, P. P. Mardilovich, and S. M. George, ConformalCoating of Ultrahigh Aspect Ratio Anodic Alumina Membranes By AtomicLayer Deposition, Chemistry of Materials 15, 3507-3517 (2003).

By repeating the binary reaction sequence in an ABAB fashion, films ofmicron thickness can be deposited with atomic layer precision. FIG. 1depicts this scenario. Moiety A is deposited onto the surface with thesubsequent deposition of moiety B. Exposing the surface to reactantmoiety A results in the self-limiting adsorption of a monolayer of the Aspecie. The resulting surface becomes the starting substrate forreaction with reactant moiety B. Subsequent exposure to moiety B coversthe surface with a monolayer of B specie. A reaction then takes placebetween the two species to form a monolayer of desired product. Anybyproducts of the surface reaction are swept away by the inert carriergas. Current viscous flow ALD reactor designs allow this monolayer bymonolayer growth to proceed very rapidly, resulting in growth rates ashigh as 1 micron per hour. ALD allows for digital control of the filmthickness at the monolayer level.

Binary reaction sequences have been developed to deposit a wide varietyof materials including oxides, nitrides, sulfides and metals. ALDcomposite oxides are formed by depositing alternating layers or partiallayers of the component oxides at a specific ratio that controls thecomposition of the composite layer. This kind of ALD system has beenreported by the inventors. J. W. Elam, M. D. Groner, and S. M. George,Viscous Flow Reactor with Quartz Crystal Microbalance for Thin FilmGrowth by Atomic Layer Deposition, Reviews of Scientific Instruments, 73(8), 2981-2987 (August 2002), and incorporated herein by reference.

The precursor moieties can be solids, liquids, or gases at roomtemperature. If necessary, the vapor pressure of the precursor moietiescan be increased by heating the precursor moieties, usually to less than200° C. A target substrate(s) is loaded into a reaction chamber througha sample loading area. The entire system is then evacuated by mechanicalvacuum pumps, which are left running through the entire process.

An inert gas such as nitrogen is allowed to flow through the system,with the system remaining at a pressure of ˜1 Torr for the duration ofthe complete deposition. Once a substrate(s) is loaded into a flow tube,and the system evacuated, a continuous gas flow is established. Thesubstrate is subsequently heated to a preselected temperature. Reactionzone temperatures range typically of from about 200° C. to 400° C. Lowertemperatures such as 100° C. can be used if the precursor moiety hassufficient vapor pressure at that temperature.

Once the preselected temperature has been attained, the first gaseousprecursor moiety is allowed to enter, as a pulse of pure gas, or with aninert gas such as nitrogen acting as a carrier, into the reaction zone.Total gas pressure is typically ˜1 Torr. The carrier gas flow rateranges from about 3 to 20 liters per hour. The precursor gas pulseduration ranges from of about one-hundredth (0.01) of a second to 10second (sec). A pulse of the first precursor moiety can be followedimmediately by a pulse of the next precursor moiety, either from thesame fluid stream or from different ingress portals. Each pulse isself-purging. In the event of pulses of pure gaseous precursor moieties,a purge pulse of inert carrier gas intervenes between pulses of pureprecursor moieties.

The inert carrier gas flow subsequently transports the precursormoieties to the reaction zone and sweeps the unused reactants andunadsorbed reaction products out of the reaction zone. Since themechanical vacuum pumps are continually running, the chemical moietiesgo through the pumps which vent into a burning box, in which thematerials are destroyed, and the box then vents into the atmosphere.

When using ALD to coat interior surfaces of anodic alumina membraneshaving a high aspect ratio L/d where L is the membrane thickness and dis the pore diameter, longer precursor exposure times are necessaryrelative to the precursor exposure times needed to coat a flatsubstrate. These longer exposure times are needed to allow the precursormolecules sufficient time to diffuse completely into the centers of thepores. The minimum precursor exposure time necessary to achieve acontinuous coating can be calculated from Equation 1.t=6.9×10⁻⁷ P ⁻¹ m ^(1/2) Ä(L/d)²  Equation 1In this formula, t is the exposure time in seconds, P is the precursorvapor pressure in Torr, m is the precursor moiety molecular weight inamu, Ä is the density of ALD reactive sites in 10¹⁵ cm⁻², and L and dhave units of nanometer (nm). This formula is taken from J. W. Elam, D.Routkevich, P. P. Mardilovich, and S. M. George, Conformal Coating ofUltrahigh Aspect Ratio Anodic Alumina Membranes By Atomic LayerDeposition, Chemistry of Materials 15, 3507-3517 (2003) at 3516, theentire reference of which is incorporated herein by reference. Theconstant in Equation 1, 6.9×10⁻⁷, is determined empirically and includesa margin of safety of a factor of 3 to account for non-idealities (i.e.,constrictions) in the AAO membrane. For example, to coat an AAO membranehaving d=10 nm and L=7×10⁴ nm (70 microns) by Al₂O₃, ALD (Ä=0.46) usingtrimethyl aluminum (TMA, m=72) at a pressure of 1 Torr requires aminimum exposure time of t=132 sec. Furthermore, as the AAO membrane islocated with successive ALD layers, the pore diameter, d, decreases andconsequently the minimum exposure time increases. The very long ALDexposure times necessary to coat the high aspect ratio AAO membranes areimpractical using the continuous flow ALD reactor scheme described suprainasmuch the precursor reservoirs are rapidly depleted, and thecontinuous pumping does not allow for high precursor vapor pressures inthe reactor flow tube. Thus, the viscous flow ALD reactor can beoperated in a quasi-static mode for the highest ratio membranesrequiring exposure times longer than approximately 10 sec.Quasi-Static Mode Example Using Al₂O₃ ALD

An exhaust valve is installed between the reactor flow tube and theexhaust mechanical pump. During each cycle for Al₂O₃ ALD, the flow tubeis first evacuated below 0.035 Torr. The exhaust valve is then closedand 1 to 10 Torr of TMA is Introduced into the reactor. After waitingfor the appropriate TMA exposure time, t, determined by Equation 1supra, the exhaust valve is opened and the flow tube is evacuated below0.035 Torr. A nitrogen purge flow of 200 standard cubic centimeters perminutes (sccm) at a pressure of 1 Torr is subsequently supplied for aperiod of 2t sec. The same sequence is repeated for the water depositionhalf-cycle using a water pressure of 1 to 10 Torr and an exposure timeof t sec. This process is repeated for each AB cycle during Al₂O₃ ALD.

The thickness of a typical deposited monolayer resulting from one ABreaction cycle is of from about 0.75 angstrom (Å) [7.5 nanometers (nm)]to 5 Å (50 nm). The thickness of a typical hybrid or reaction productlayer is of from about 1.5 Å (15 nm) to 10 Å (100 nm). Specificthicknesses of layers are dependent, however, upon the nature of thedeposited substance.

The following example is only to illustrate how a reaction is carriedout between two precursors to leave a monolayer of product on asubstrate, e. g., one method of depositing a monolayer of alumina,Al₂O₃, on a substrate such as AAO.

Deposition Example

Consider the following binary A-B reaction cycle, illustrated byEquations 2 and 3, for the ALD of alumina (Al₂O₃) via the reaction oftrimethyl aluminum (TMA) with hydroxyl (OH).Reaction A I—Al—OH*+Al(CH₃)₃→I—Al—O—Al(CH₃)₂*+CH₄  Equation 2Reaction B I—Al—O—Al(CH₃)₂*+H₂O→I—Al—O—Al—O—OH*+CH₄  Equation 3

In Equations 2 and 3, the asterisks designate moieties adsorbed to thesubstrate surface, the I— indicates the substrate surface, and theequations have been simplified to show only one surface active site. Theactual scheme involves several active sites at once. In Equation 2, thesubstrate (e. g., AAO) surface is initially covered with hydroxyl (OH)moieties deposited by exposure of the substrate surface to water. Thehydroxyl moieties react with TMA to deposit a monolayer of aluminumatoms that are terminated by methyl (CH₃) species, and releasing methane(CH₄) as a reaction byproduct. This methane can be shunted to areclamation system to protect the system. TMA is not reactive to themethyl termini protruding from the now covered surface. Thus, due to themethyl termini, additional exposure of this surface to TMA gives noadditional growth on the surface beyond the one monolayer alreadypresent on the surface.

In Equation 3, subsequent exposure of this new surface to waterdisplaces the methyl moiety, and leaves a hydroxyl in its place. Thehydroxyl reacts with a pulse of fresh TMA and creates another monolayerof Al—O ionic bonds. Methane is once again released as a byproduct. Thenet effect of one AB cycle is to deposit one monolayer of alumina on thesubstrate surface. Multiple cycles produce multiple layers.

FIG. 2A displays a secondary electron micrograph of an AAO membrane,with pore diameters of approximately 40 nm, before ALD deposition. Thepores go completely through the thickness of the membrane. FIG. 2Bdisplays the same membrane with pores of diameter 10 nm, afterdeposition of a 15 nm layer of Al₂O₃ by ALD. The holes in FIGS. 2A and2B are the pores.

Temperatures of about 250° C. are preferred for depositing titania(TiO₂) films on flat surfaces. A reaction zone temperature of 250° C. inthat situation produces denser and more crystalline films.

However, while reaction zone temperatures of from about 200° C. to 400°C. have been disclosed herein, the inventors have found that lowerreaction zone temperatures can give optimal results. Specifically, theinventors have developed methods for depositing conformal TiO₂ filmsinside of the AAO substrate nanopores using alternating exposures totitanium tetrachloride and water at a deposition temperature of 100 C.For ALD of titania in AAO, a lower reaction zone deposition temperatureof about 100° C. allows for the formation of amorphous titania filmswhich are smoother than crystalline anatase films, the later of whichform at 250° C. The lower deposition temperature serves as a means toprevent formation of large anatase TiO₂ nanocrystals which cause surfaceroughness and block the AAO nanopores to further gas transport. Thesesmoother amorphous titania films aid in keeping the AAO nanopores opento gas transport and gas throughput. Even at this lower temperature of100° C., nucleation of titania via ALD occurs readily on the ALD Al₂O₃surface. Accordingly, titania formation occurs without delay.

The inventors have also developed methods for depositing conformalvanadia (V₂O₅) films by ALD using alternating exposures to vanadyltrisisopropoxide (VO[(CH₃)₂CHO]₃ and a 30% aqueous solution of hydrogenperoxide (H₂O₂) at a reaction zone temperature of approximately 100° C.These conditions yield an ALD vanadia growth rate of 0.41 Å/(A-B)exposure cycle which is higher than the growth rate using alternating(A-B) vanadyl trisisoproxide (VO[(CH₃)₂CHO]₃/water exposures. Thisdeposition method serves as a means for depositing small (less than 2-3Angstrom) sub-monolayer quantities of catalytic V₂O₅ on the TiO₂catalytic support layer.

Catalyst Example

An approximate monolayer of vanadium oxide was formed on theaforementioned AAO membrane by impregnation to a loading of 12micromoles of vanadia per square meter (V₂O₅/m²). This flow-throughmembrane catalyst was compared with two conventional V₂O₅ catalystssupported on a high surface area alumina for the oxidativedehydrogenation of cyclohexane at 450° C. The 20 wt. % V₂O₅/Al₂O₃catalyst has a similar monolayer coverage of vanadia (12 micromoleV₂O₅/m²), and the 1 wt. % V₂O₅/Al₂O₃ catalyst was chosen to obtain aconventional vanadia catalyst with a conversion rate comparable to theconversion rate of the vanadia-AAO membrane catalyst.

As shown in Table 1 infra, the AAO-supported V₂O₅ catalyst exhibitedmuch higher selectivity for the partial oxidation product, cyclohexene,than either of the conventional catalysts. The inventors surmise thisreflects the short contact time of the reagents in the flow-throughmembrane channels which limits the secondary oxidation reactions whichwould produce benzene (C₆H₆), carbon monoxide (CO), and carbon dioxide(CO₂). The reagents have only one quick pass at/on the catalyticmaterial as the reagent molecules will most likely pass through thecatalytic pores.

The V₂O₅/AAO catalyst exhibits exceptionally high selectivity toward thefirst oxidation product, cyclohexene, compared to the 1% V₂O₅/Al₂O₃catalyst for the same conversion. The inventors surmise this reflectsthe comparative absence of secondary reactions in the AAO-basedcatalyst, and thus the selectivity of the catalyst. TABLE 1 Conversionand Selectivity for catalytic oxidation of cyclohexane by conventionaland AAO supported V2O5 catalysts^(a) 1% 20% 0.1% V₂O₅/Al₂O₃ V₂O₅/Al₂O₃V₂O₅/Al₂O₃/AAO Surface density^(b) 0.6 12 12 Flow rate^(c) 100 100 50O₂:Cyclohexane Ratio 1.5 1.6 1.2 Total Conversion (%) 3.3 32.4 3.2Cyclohexene (%)^(d) 28.5 / 56.0 Benzene (%)^(d) 12.2 36.4 / CO (%)^(d)25.3 22.5 14.6 CO₂ (%)^(d) 33.9 41.1 29.3^(a)at 450° C.^(b)micromole of V₂O₅/m².^(c)milliliters per minute (ml/min).^(d)specie percentage of total conversion.

V₂O₅/AAO is an active catalyst and formation of products can be detectedafter a single pass of the reagents through the membrane and withoutconcentrating the product in a cold trap.

The pores of anodized aluminum are reduced to sizes less than or equalto 10 nm by the deposition of metal oxide films with ALD. The electronicstructure of materials changes in the 1 nm to 10 nm range.

Flow-through catalytic membranes with pores acting as reactant channelscan be made.

The flow-through feature can allow for control of the extent of reactionsuch as partial hydrogenation of reactant species as opposed to completehydrogenation.

The pore diameter reduction allows tailoring of channel size and thelayer deposition allows alteration of wall composition.

The invention uses a process which is a combination of anodic aluminumoxidation and ALD.

There are no physical limitations on the types of catalysts which can bedeposited on coated AAO.

Porous catalysts can be fabricated which allow for atomic level controlof both the pore wall diameters and pore wall composition.

There is uniform coating of the entire length of the pore inner surface.Pores of uniform and consistent diameter can be produced. This allowsfor consistently identical results throughout the catalyst.

The process's operating temperature is typically of from about 200° C.to 400° C., but can be used at lower temperatures if the precursormoieties's vapor pressures so permit. A large number of different typesof catalysts can be generated. The invention can provide unique catalystenvironments.

While the invention has been described with reference to details of theillustrated embodiments, these details are not intended to limit thescope of the invention as defined in the appended claims.

1. A process for making a membrane catalyst having pores at no greaterthan 10 nm diameters, the process comprising: a) depositing alternatingmonolayers of different precursor moieties upon a substrate; b) allowingthe monolayers to react with each other to form a first film upon thesubstrate; c) depositing a second group of alternating monolayers of thedifferent precursor moieties upon the first film; and d) allowing themonolayers from the second group to react and form a second film uponthe first film.
 2. The process as recited in claim 1 wherein thesubstrate is anodic aluminum oxide.
 3. The process as recited in claim 1wherein precursor moieties are selected from the group consisting oftitanium tetrachloride, titanium (IV) isopropoxide, tin (IV) chloride,tetraethyl tin, trimethyl aluminum, silicon tetrachloride, vanadium (V)isopropoxide oxide, chromium (III) acetyl acetonate, nickel (II) acetylacetonate, vanadyl acetyl acetonate, bis(cyclopentadienyl) ruthenium,methylcyclopentadienyltrimethyl platinum, and combinations thereof. 4.The Process as recited in claim 1 wherein the pore diameter of thesubstrate is the size of the smallest precursor moiety.
 5. The processas recited in claim 1 wherein one of the precursor moieties is selectedfrom the group consisting of water, oxygen, ozone, hydrogen peroxide,isopropanol, air, and combinations thereof.
 6. The process as recited inclaim 1 wherein a first monolayer for the first film is deposited, thefirst monolayer comprising a precursor moiety selected from the groupconsisting of titanium tetrachloride, titanium (IV) isopropoxide, tin(IV) chloride, tetraethyl tin, and trimethyl aluminum, silicontetrachloride, and combinations thereof.
 7. The process as recited inclaim 1 wherein a second monolayer for the first film is deposited, thesecond monolayer selected from the group consisting of water, oxygen,ozone, hydrogen peroxide, isopropanol, air and combinations thereof. 8.The process as recited in claim 1 wherein the first film is selectedfrom the group consisting of aluminum oxide (alumina/Al₂0₃), titanium(IV) oxide (titania/TiO₂), tin (IV) oxide (stannia/SnO₂), and silicon(IV) oxide (silica/SiO₂).
 9. The process as recited in claim 1 wherein afirst monolayer for the second film is deposited, the first monolayercomprising a precursor moiety selected from the group consisting ofvanadium (V) oxide (vanadia/V₂O₅), chromium (III) acetyl acetonate(Cr(C₅H₇O₂)₃, nickel (II) acetyl acetonate (Ni(C₅H₇O₂)₂), vanadyl acetylacetonate (VO(C₅H₇O₂)₂), bis(cyclopentadienyltrimethyl platinum((C₆H₇)Pt(CH₃)₃).
 10. The process as recited in claim 1 wherein a secondmonolayer for the second film is deposited, the second monolayerselected from the group consisting of water, oxygen, ozone, hydrogenperoxide, isopropanol, air and combinations thereof.
 11. The process asrecited in claim 1 wherein the second film is selected from the groupconsisting of vanadium (V) oxide (vanadia/V₂O₅), chromium oxide, nickel(Ni), platinum (Pt), and ruthenium (Ru).
 12. The process as recited inclaim 1 wherein the process is carried out at temperatures of from about100° C. to 400° C.
 13. A porous support material for a catalystcomprising a film deposited upon anodized aluminum oxide so as to formuniform pores having diameters no greater than 10 nm.
 14. The poroussupport material recited in claim 13 wherein the film is selected fromthe group consisting of aluminum oxide (alumina/Al₂O₃), titanium (IV)oxide (titania/TiO₂), tin (IV) oxide (SnO₂), and silicon (IV) oxide(silica/SiO₂).
 15. A catalytic substrate comprising a catalytic layercoating upon a porous support material.
 16. The substrate as recited inclaim 15 wherein the layer is selected from the group consisting ofvanadium (V) oxide (vanadia/V₂O₅), chromium oxide (CrO_(x)), nickel,platinum, ruthenium, and combinations thereof.
 17. The substrate asrecited in claim 15 wherein the catalytic layer is deposited on theinner surfaces of pores of the support material.
 18. The substrate asrecited in claim 15 wherein the catalyst coated pores have diametersless than 10 nm.
 19. The substrate as recited in claim 15 wherein thepores traverse the substrate to enable a flow-through catalyst.
 20. Thesubstrate as recited in claim 18 wherein the pores have a pore sizedistribution within 0.1 nm with predetermined catalyst precursormoieties.
 21. A porous separation membrane comprising multiple layers offilm deposited upon anodized aluminum oxide via atomic layer deposition.22. The membrane as recited in claim 21 wherein pores within themembrane are the size of a particular molecule to be separated from afluid stream.